The invention relates to an electroluminescent device comprising an active layer having an electroluminescent property, which layer includes 2,5-substituted poly(p-phenylene vinylene) and which is situated between two electrode layers, at least one of said electrode layers being transparent to the light to be emitted and, in operation, said device exhibiting a light-emitting pattern. The invention also relates to a method of manufacturing an electroluminescent (EL) device having a structured active layer.
The active layer and both electrode layers can jointly form one light-emitting diode (LED), but the EL device preferably comprises various LEDs, for example in the form of a matrix of light-emitting surfaces, as intended for a display. An EL device emits light when an electric field is applied across the active or emissive layer. Such a device cannot only be used as a display but also, for example, as a light source.
The use of inorganic materials such as GaAs for the active layer has been known for a long time. Since a few years also organic materials are known, in particular semiconductive organic polymers, which can be used for the active layer. Semiconductive organic polymers have a conjugated polymer chain. A well-known polymer is, for example, poly(p-phenylene vinylene) (PPV), in particular 2,5-substitued PPV. The band gap, the electron affinity and the ionization potential can be adjusted by choosing the proper conjugated polymer chain and proper side chains. Unlike electrically conducting polymers, these conjugated polymers are undoped. In addition, such polymers enable flexible substrates to be used. The active layer of an organic polymer is situated between two electrode layers of electrically conducting materials, namely one for the injection of holes and one for the injection of electrons into the active layer.
Both electrode layers can be structured in accordance with a pattern by means of a customary photolithographic process, so that the desired pattern for a display or, for example, for an illuminated logo is obtained. In a typical example of a display, the electrodes of the first and the second electrode layers have line structures which intersect each other at right angles and hence form a matrix of individually addressable rectangular LEDs. Said rectangular LEDs form the pixels or picture elements of the display. If the electrodes of the first and the second electrode layers are connected to a voltage source, light-emitting pixels are formed at the intersections of the electrodes.
Such an EL device is known from the international Patent Application WO 90/13148. In said known EL device both electrode layers are provided in the form of strips intersecting at right angles by means of a vacuum-evaporation process. For this purpose use is made of a mask during said vacuum-evaporation process. A drawback of said known method is that both electrode layers must be structured in order to cause individual pixels to luminesce.